JP2008531298A - System and method for processing nanowires with holographic optical tweezers - Google Patents

Abstract

  Systems and methods for manipulating and processing nanowires in a liquid with an array of holographic light traps. The system and method of the present invention can generate hundreds of individually controlled light traps with the ability to manipulate objects in three dimensions. Individual nanowires with cross-sections as small as 20 nm and lengths exceeding 20 μm are separated, translated and rotated by an array of holographic optical traps under conditions such that a single trap has no discernable effect. And can be deposited on the substrate. Spatially localized photothermal and photochemical processes induced by focused traps can also be used to melt localized regions of individual nanowires and fuse nanowire junctions.

Description

  This application is an application claiming the advantages of US Provisional Patent Application No. 60 / 643,384, filed on January 12, 2005, based on section 119 (e) of the US Patent Act. Incorporated herein by reference.

  This study was supported by the National Science Foundation with grant numbers DMR-0233971 and DBI-0450878.

[Field of the Invention]
The present invention relates generally to semiconductor and metal nanowires. More particularly, the present invention relates to techniques for manipulating and processing semiconductor and metal nanowires in solution with an array of holographic light traps.

  Semiconductor and metal nanowires are one-dimensional structures with unique electrical and optical properties that are used as building blocks in nanoscale devices. Their low dimensionality means that they exhibit quantum confinement effects. For this reason, and for other reasons, such nanowires are versatile building blocks in assembling functional electronic and photonic devices. Realizing their potential requires an efficient way to assemble them into complex and specially configured structures.

  Accordingly, it is an object of the present invention to provide an improved system and method for manipulating semiconductor and metal nanowires.

  Another object of the present invention is to provide an improved system and method for increasing the amount of force that can be applied to semiconductor and metal nanowires while minimizing radiation damage.

  Additional embodiments of the present invention provide improved systems and methods for translating semiconductor and metal nanowires.

  In accordance with the above objectives and other objectives described herein below, the present invention provides a system and method for assembling nanowires into precisely organized 2D and 3D structures using an array of holographic optical tweezers. including. Individual nanowires with cross sections as small as 20 nm and lengths greater than 20 μm are separated, translated and rotated by an array of holographic optical traps, provided that the individual traps have no discernable effect. Or otherwise manipulated or deposited on a substrate. Spatially localized photothermal and photochemical processes induced by focused traps can also be used to melt localized regions of individual nanowires and fuse nanowire junctions.

  These and other objects, advantages and features of the present invention, as well as its construction and method of operation, will become apparent from the following detailed description taken in conjunction with the accompanying drawings. In the drawings, similar elements have similar designations throughout the several figures described below.

The present invention provides systems and methods for manipulating and processing nanowires in solution with an array of holographic light traps. In certain embodiments of the invention, CdS and Si nanowires are dispersed in water to practice the invention. In this particular embodiment, CdS nanowires have a nominal diameter of 80 nm and a length up to 20 μm, while Si nanowires have diameters as small as 20 nm and an even larger aspect ratio. These samples are loaded into slit pores approximately 40 μm thick formed with a clean cover glass edge on the surface of the microscope slide. Both materials are substantially denser than water (ρ _CdS = 4.8 g / cm ³ , ρ _Si = 23 g / cm ³ ) and settle rapidly on the lower glass wall, almost complete for CdS samples Located in the plane. In this embodiment of the invention, the sealed sample is mounted on the stage of a Zeiss Axiovert S100-TV microscope equipped with a 100x, NA1.4 S-Plan Apo oil immersion objective for observation and manipulation. Is done. This lens focuses light from a continuous wave (CW) frequency doubled Nd: YVO ₄ laser (Coherent Verdi) operating at 532 nm into an optical trap to generate a bright field image of dispersed nanowires Also used for.

  A precisely focused single light beam forms an optical trap, known as optical tweezers, that can capture mesoscopic objects in three dimensions. However, at laser powers below about 1 W, it appears that individual optical tweezers cannot move any type of semiconductor nanowire. Due to rapid heating at higher power, vapor bubbles are generated when the focal point passes through the nanowire. Such rapid heating also leads to visible changes in the nanowire itself, including bending, nodule formation, and even cutting. This is consistent with the heating due to the large light absorption caused by the photon flux passing through the focal volume.

  A number of diffraction-limited optical traps are projected using dynamic holographic optical tweezer technology to apply more force to the nanowire while minimizing radiation damage. This approach uses a spatial light modulator (SLM) (Hamamatsu X7550 PAL-SLM) to transform a laser-wavefront of a computer-designed phase-only hologram that encodes the desired array of traps before focusing. Engrave on. Each trap in the array can be independently translated in three dimensions by projecting a series of holograms that encode a sequence of intermediate trap configurations.

  The image in FIG. 1 (a) shows the focused light from an array of linear 60 traps moving the CdS nanowire in FIG. 1 (b). In this case, the array was focused within 0.5 μm from the surface of the nanowire, resulting in an output of 3 mW per trap. Initially, nanowires oriented perpendicular to the array of optical traps rotate and align within a few seconds, even at such relatively low laser power. Assuming a 0.4 μm inter-trap distance, approximately 15 traps are simultaneously aimed at this nanowire in its final configuration.

ここでε＝［ｌｎ（Ｌ／Ａ）］^-1であり、それは、この断面積のＣｄＳナノワイヤに対し１トラップ当たり０．２ｆＮの光学的に加えられる力の下限を設定する。このナノワイヤに対する実際の抗力は、ナノワイヤの中心からｈ≒０．５μｍの距離の境界ガラス表面によって実質的に強化され、次式によって最低次数のａ／ｈで与えられる。

これは捕獲力の推定値を少なくとも２倍に増大させる。 Given the above conditions, once the nanowires are aligned with the array, they can be translated at a speed of up to approximately u = 10 μm / sec by moving the array within the field of view or by moving the sample stage relative to the array. Can be made. Note that nanowires can be translated at speeds exceeding u = 10 μm / sec by any of a variety of other mechanisms to increase power, optimize laser wavelength, or increase effective force per trap. I want to be. The drag force of a cylinder having a length L and a radius a that translates in an infinite fluid having a low Reynolds number and a viscosity η is approximately as follows.

Where ε = [ln (L / A)] ⁻¹ , which sets the lower limit of the optically applied force of 0.2 fN per trap for this cross-sectional CdS nanowire. The actual drag on this nanowire is substantially enhanced by the boundary glass surface at a distance h≈0.5 μm from the center of the nanowire, and is given by the lowest order a / h.

This increases the capture power estimate by at least a factor of two.

  The above estimate suggests that a single optical tweezer should be able to move the nanowire. However, even if so, the nanowire rotates in a direction that minimizes drag in the direction of motion and therefore escapes from the trap. The spatially extended capture potential provided by the array of holographic optical tweezers maintains the orientation of the nanowire and thus allows controlled translation.

をＳＬＭで捕獲用レーザの波面に刻み付けることによって個々のトラップが光渦（optical voltex）に変換されるときに、さらによく実証される。この状況で、

は、ビームの軸線に対しビームに垂直な面内の極座標であり、

は波面のヘリシティを定義する整数の巻数である。この変調の効果は、点状光ピンセットを、半径が巻数と線形的に縮尺されるリング状トラップに変形することであり、その光子は各々それらの固有スピン角運動量に加えて、

の軌道角運動量を運ぶ。この角運動量は、光のリングを照射される物体に伝達することができ、結果的に光の強度に比例する正味トルクが生じる。 The traditional nature of nanowire response to optical tweezers is the helical phase profile

This is even better demonstrated when individual traps are converted into optical vortices by engraving them into the wavefront of a capture laser with an SLM. In this situation,

Is the polar coordinate in the plane perpendicular to the beam relative to the axis of the beam,

Is an integer number of turns defining the helicity of the wavefront. The effect of this modulation is to transform the point-like optical tweezers into a ring-shaped trap whose radius is linearly scaled with the number of turns, each of which photons in addition to their intrinsic spin angular momentum,

Carry orbital angular momentum. This angular momentum can be transmitted to the object illuminated by the ring of light, resulting in a net torque proportional to the light intensity.

  Both Si and CdS nanowires themselves tend to align in the tangential direction of the ring of light. Once oriented tangentially, the cross-sections subjected to the radiation pressure become larger and they are displaced in the radial direction. Nevertheless, while in the region of the optical vortex trap, the nanowire is driven around the periphery in the same direction as a conventional micrometer-scale dielectric sphere. These observations are consistent with the interpretation that nanowires are subject to the action of optical tweezers as a trap for conventional optical gradient forces, despite their very small cross-sectional area.

  Optical tweezers can also move single nanowires vertically along the optical axis and press them against the substrate. This results in the nanowires being irreversibly anchored to the substrate by van der Waals interactions if no special care is taken to stabilize the nanowires from depositing. If nanowires are stabilized by, for example, a single layer of adsorbed polymer surfactant, they can still be fixed in place by selective photochemistry or photothermal processes. These simplest methods involve increasing the laser power until the stabilization layer is desorbed or destroyed. Such selective contact deposition can provide a basis for controlling and assembling nanowires on prefabricated functional substrates. More aggressive forms of such optical processing can be used to selectively melt the contacts between nanowires, thus fusing them into a permanent structure. More accurate variations can use linear or non-linear photochemical processes to selectively induce photochemical changes at the nanowire junction so that specific functionality is generated.

  Nanowires of the type described herein can also be modified by irradiating the nanowire with a specific intensity or wavelength of light. Each intensity and wavelength is selected to affect certain changes along the length of the nanowire. Changes that can be achieved include nanowire melting, nanowire cutting, and chemical transformations. All of these changes can occur at the junction between nanowires composed of similar or different materials. Such a conversion can result in mechanical, electrical or optical contact not only between nanowires, but also between nanowires and other substrates.

  The results presented here demonstrate that an array of holographic optical tweezers can be used to assemble semiconductor nanowires into precision organized 2D and 3D structures. This process can be optimized by adjusting the laser wavelength to enhance the light capture power, significantly speeding up and making significant progress in parallel with advances in holographic capture technology.

  The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description. It is not exhaustive and is not intended to limit the invention to the precise form disclosed, and may be modified and varied in light of the above teachings or may be derived from practice of the invention. It may be. This embodiment illustrates the principles and practical application of the present invention so that one skilled in the art can utilize the present invention in various embodiments and with various modifications to suit the particular application envisaged. Selected and described for explanation.

FIG. 6 is an image showing a focused light beam from an array of linear 60 traps. FIG. 6 is an image showing multiple exposure of a single CdS nanowire rotated and translated by an array of light traps.

Claims (11)

Inputting a plurality of light beams;
Forming a plurality of optical traps;
Projecting the plurality of optical traps onto a first nanowire;
Treating the at least first nanowire; and treating the nanowire.   The process of claim 1, wherein the step of processing the at least first nanowire comprises at least one of manipulating, separating, heating, and chemically converting the at least first nanowire. Method.   The method of claim 1, wherein the at least first nanowire comprises at least one of a metal nanowire and a semiconductor nanowire.   The method of claim 1, wherein the at least first nanowire comprises at least one of CdS and Si.   The method of claim 1, further comprising translating the at least first nanowire by changing a position of the array of optical traps with respect to the at least first nanowire. Providing a second nanowire;
The method of claim 1, further comprising fusing the at least first nanowire to the second nanowire by increasing the power projected by at least one of the array of light traps. Converting at least one of the plurality of optical traps into at least one optical vortex;
The method of claim 1, further comprising: radially translating the at least first nanowire using the at least one optical vortex.   The method of claim 1, wherein the plurality of light beams have a predetermined wavelength to effect at least a partial change in the at least first nanowire.   The method of claim 1, wherein the plurality of light beams have a predetermined intensity to cause at least a partial change in the at least first nanowire.   The method of claim 1, wherein the at least first nanowire and the second nanowire are selected from the group of the same material and different materials. The change forms a contact between the at least first nanowire and the second nanowire, the second nanowire and non-nanowire substrate, and at least two of the at least first nanowire and non-nanowire substrate. Is,
The method of claim 1, wherein the contact is selected from the group consisting of a mechanical contact, an electrical contact, and an optical contact.

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